A Hybrid Mean Value Involving the Two-Term Exponential Sums and Two-Term Character Sums
The main purpose of this paper is using the properties of Gauss sums and the estimate for character sums to study the hybrid mean value problem involving the two-term exponential sums and two-term character sums and give an interesting asymptotic formula for it.
Let be an integer and denotes a Dirichlet character . For any integers and with , we define the two-term exponential sum and two-term character sum as follows: where , denotes a nonprincipal Dirichlet character , and is a fixed positive integer.
These sums play a very important role in the study of analytic number theory, so they caused many number theorists’ interest and favor. Some works related to can be found in [1–5]. For example, Cochrane and Zheng  show that where denotes the number of all distinct prime divisors of .
On the other hand, the sums are a special case of the general character sums of the polynomials where and are any positive integers and is a polynomial. If is an odd prime, then Weil (see ) obtained the following important conclusion.
Now we are concerned about whether there exists an asymptotic formula for the hybrid mean value
In this paper, we will use the analytic method and the properties of character sums to study this problem and give a sharp asymptotic formula for (5) with , an odd prime. That is, we will prove the following.
Theorem 1. Let be an odd prime, let be any nonprincipal even character , and let be the principal character . Then we have the asymptotic formula where satisfies the inequalities .
From this theorem we may immediately deduce the following.
Corollary 2. For any odd prime and any nonprincipal even character with , one has
In the theorem, we only consider the polynomial . For general polynomial with and , whether there exists an asymptotic formula is complex problem for (5), it needs us to further study.
For general positive integer , whether there exists an asymptotic formula for (5) is also an interesting open problem.
2. Several Lemmas
To complete the proof of our theorem, we need the following several lemmas.
Lemma 1. Let be an odd prime and let be any nonprincipal even character . Then for any integer with , the identity where denotes the Legendre symbol and .
Proof. Since , there exists one and only one character such that . Thus, from the properties of Gauss sums we have
This proves Lemma 1.
Lemma 2. Let be an odd prime, let be any nonprincipal even character , , and , the principal character . Then for any integer and any quadratic nonresidue with , we have the identity
Lemma 3. Let be an odd prime, let be any nonprincipal even character , , and , the principal character . Then for any integer and any quadratic nonresidue with , one has the estimate
Lemma 4. Let be a prime. Then we have the identity where denotes the Legendre symbol.
Proof. For any odd prime and integer with , from Hua's book  (Section 7.8, Theorem 8.2) we know that
From this identity and the definition and properties of Gauss sums we have
This proves Lemma 4.
3. Proof of the Theorem
In this section, we will complete the proof of our theorem. Note that the identities and
This completes the proof of our theorem.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the referee for their very helpful and detailed comments, which have significantly improved the presentation of this paper. This work is supported by the NSF (11371291) of China.